1. Field of the Invention
The present invention relates to a method for manufacturing a magnetoresistive element that is used in a magnetic sensor, a magnetic head, a magnetoresistive memory (a magnetic random access memory, referred to as xe2x80x9cMRAMxe2x80x9d), etc.
2. Description of the Related Art
A magnetoresistance effect is a phenomenon in which electrical resistance changes by the application of a magnetic field to a magnetic material. A multilayer film having a structure in which a magnetic layer and a non-magnetic layer are stacked alternately (i.e., magnetic layer/non-magnetic layer/magnetic layer/non-magnetic layer/ . . . ) can provide a large magnetoresistance effect known as a giant magnetoresistance (GMR) effect. For a GMR element, a conductive layer made of Cu, Au, etc. is used as the non-magnetic layer. A GMR element that allows a current to flow parallel to the film surface is called a CIP-GMR (current in plane-GMR) element. A GMR element that allows a current to flow perpendicular to the film surface is called a CPP-GMR (current perpendicular to the plane-GMR) element. The CPP-GMR element has a larger magnetoresistance change ratio (MR ratio) and a smaller resistance value compared with the CIP-GMR element.
A spin valve element is one of the magnetoresistive elements that does not require a large operating magnetic field. This element includes a free magnetic layer and a pinned magnetic layer that sandwich a non-magnetic layer. The spin valve element utilizes a change in relative angle formed by the magnetization directions of the two magnetic layers that is caused by magnetization rotation of the free magnetic layer. As an example of the spin valve GMR element, an element in which a magnetization rotation control layer made of an antiferromagnetic material (FeMn) is stacked on a Nixe2x80x94Fe/Cu/Nixe2x80x94Fe multilayer film has been proposed. Although this element requires a smaller operating magnetic field and has excellent linearity, the MR ratio is low. Another spin valve GMR element has been reported that improves the MR ratio by using a CoFe ferromagnetic material for the magnetic layer and PtMn and IrMn ferromagnetic materials for the antiferromagnetic layer.
To achieve a higher MR ratio, an element that uses an insulating material for the non-magnetic layer and allows a current to flow perpendicular to the film surface has been proposed as well. This element can provide a so-called tunnel magnetoresistance (TMR) effect by statistically transmitting a tunnel current through the non-magnetic layer (tunnel layer) that serves as an insulating layer. A higher MR ratio can be expected from the TMR element as the spin polarization of the magnetic layers sandwiching the insulating layer is increased. Therefore, magnetic metals such as Fe, Fexe2x80x94Co alloy and Nixe2x80x94Fe alloy, a half-metallic ferromagnetic material, or the like are suitable for the magnetic layers.
There also have been studies on an MRAM device that is produced by forming a magnetoresistive element on CMOS. Such a CMOS process includes high-temperature heat treatment at 400xc2x0 C. to 450xc2x0 C. However, the heat treatment at not less than 400xc2x0 C. reduces the MR ratio of the magnetoresistive element.
It is an object of the present invention to provide a magnetoresistive element that can suppress the characteristic degradation even after high-temperature heat treatment, specifically at 400xc2x0 C. to 450xc2x0 C.
The present invention provides a method for manufacturing a magnetoresistive element. The magnetoresistive element includes a substrate and a multilayer film formed on the substrate. The multilayer film includes a first ferromagnetic layer, a second ferromagnetic layer, and a non-magnetic layer arranged between the first ferromagnetic layer and the second ferromagnetic layer. A resistance value changes with a change in relative angle formed by the magnetization directions of the first ferromagnetic layer and the second ferromagnetic layer. The manufacturing method of the present invention includes the following: a film formation process for forming at least the first ferromagnetic layer, the second ferromagnetic layer, and the non-magnetic layer on the substrate; a preheat process at 330xc2x0 C. to 380xc2x0 C. for not less than 60 minutes, e.g., for 60 to 300 minutes, and preferably for 60 to 180 minutes performed after the film formation process; and a heat treatment process at 400xc2x0 C. to 450xc2x0 C. performed after the preheat process.